1. Field of the Invention
The present invention relates to a current source power converting apparatus with an improved overvoltage protection, and especially to an improvement of an overvoltage protection of a current source power converting apparatus having an AC to DC converter and a DC to AC inverter coupled with the converter through a DC reactor.
2. Description of the Related Art
Recently, a current source power converting apparatus has widely been employed in order to control an AC load, for example an induction motor. As is well known, a current source power converting apparatus has an AC to DC converter and a DC to AC inverter, in which the AC to DC converter is composed of semiconductor switching elements which repeat turning on or off to convert AC power supplied by an AC power source into DC power and the DC to AC inverter is also composed of semiconductor switching elements which repeat turning on or off to reconvert the DC power into AC power to be supplied for an AC load such as an induction motor. Further, there is provided a DC reactor between the converter and the inverter, whereby a constant DC current is supplied for the inverter.
In such a current source power converting apparatus, when the switching elements of the converter or the inverter are turned off during of the switching operation thereof, or when any of the switching elements, through which current is now flowing, is abnormally made off, current flowing through a circuit of the power converting apparatus is abruptly interrupted and thereby a high voltage is induced by inductance existing in the circuit. Inductance exists in an AC side of the circuit, i.e. in an AC power source and an AC load, as well as the DC reactor. The high voltage thus induced is applied to circuit components and, especially, when such a high voltage becomes an overvoltage of the switching elements they may be destroyed.
Conventionally, there has not been proposed an appropriate protection for a current source power converting apparatus as described above, which can effectively prevent semiconductor switching elements from being destroyed due to the overvoltage. For example, in an article entitled "Configuration and Characteristics of the GTO Converter Using Regenerative Voltage Clipper Circuit" by Ise et al (Journal of Institute of Electric Engineers of Japan, Vol. 106 No. 9 (1986) p.p. 25 to 32), there is disclosed an overvoltage protection of gate turn-off thyristors of a converter, which controls current supplied for a coil of a superconductive magnetic energy by converting AC power into DC power.
In such a converter, a high voltage is induced by leakage inductance on the AC side or that of the coil, when GTO is turned off. According to the aforesaid prior art, the induction of such a high voltage is suppressed by absorbing the energy stored in the inductance by an energy absorbing circuit coupled to a DC side of the converter. When GTO is turned off, the energy stored in the inductance is discharged to make current circulate through the energy absorbing circuit, the coil (load), the converter and the AC power source, with the result that during the circulation of the current, the energy is absorbed by the energy absorbing circuit.
As disclosed in the prior art mentioned above, in the case where a load such as coil is coupled to a converter, current for discharging the energy stored in inductance of a circuit can always circulate automatically through the load, when GTO is turned off or when GTO is made off abnormally. If, however, an inverter is coupled to the converter as a load, such current can circulate only when the inverter operates normally. If GTOs of both the converter and the inverter are made off abnormally at the same time, a circulation circuit for discharging the stored energy is no longer formed. If the stored energy can not discharged, a high voltage is induced and applied to GTOs, so that they may be destroyed due to an overvoltage. The prior art mentioned above does not provide any effective countermeasures thereagainst.